The following references are considered to be pertinent for the purpose of understanding the background of the present invention:
(1) Hutter, E.; Fendler, J. H. Exploitation of localized surface plasmon resonance, Adv. Mater. 2004, 16, 1685.
(2) Kalyuzhny, G.; Vaskevich, A.; Schneeweiss, M. A.; Rubinstein, I. Transmission surface-plasmon resonance (T-SPR) measurements for monitoring adsorption on ultrathin gold island films, Chem.-Eur. J. 2002, 8, 3850.
(3) Doron-Mor, I.; Barkay, Z.; Filip-Granit, N.; Vaskevich, A.; Rubinstein, I. Ultrathin Gold Island Films on Silanized Glass. Morphology and Optical Properties., Chem. Mater. 2004, 16, 3476.
(4) Doron-Mor, I.; Cohen, H.; Barkay, Z.; Shanzer, A.; Vaskevich, A.; Rubinstein, I. Sensitivity of transmission surface plasmon resonance (T-SPR) spectroscopy: Self-assembled multilayers on evaporated gold island films, Chem.-Eur. J. 2005, 11, 5555.
(5) Wanunu, M.; Vaskevich, A.; Cohen, S. R.; Cohen, H.; Arad-Yellin, R.; Shanzer, A.; Rubinstein, I. Branched Coordination Multilayers on Gold, J. Am. Chem. Soc. 2005, 127, 17877.
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Various sensors making use of surface plasmon resonance such as biosensors, gas sensors, concentration sensors and ion sensors have been proposed in recent years.
The coupling of electromagnetic radiation to metal island films results, under certain condition, in enhanced extinction due to localized surface plasmon resonance (LSPR) scattering. Ultrathin gold films, prepared by evaporation of sub-percolation layers (typically up to 10 nm nominal thickness) onto transparent substrates followed by optional annealing, form arrays of well-defined metal islands with tunable wavelength of surface plasmon (SP) absorption band. As the SP band intensity and position are sensitive to the film morphology (island shape and inter-island separation) and the effective dielectric constant of the surrounding medium, such systems can be used for chemical and biological sensing in the transmission localized surface plasmon resonance (T-LSPR) mode. The localized SP coupling observed in dense assemblies of nanoparticles (NPs), either in solution or immobilized on solid substrates, introduces an additional factor which may strongly influences the conditions of the SP resonance.
Sensors based on localized SP resonance can be divided into two broad groups, namely, sensors based on monitoring changes in the dielectric constant of the immediate environment, and sensors based on changes in SP coupling. A combination of these effects was also exploited in localized SP sensing [1].
Changes in the localized SP band of discontinuous Au or Ag films upon analyte binding can be conveniently monitored using standard spectrophotometric equipment in the transmission mode, presenting a notable advantage in sensing applications. This method of transduction was termed transmission localized surface plasmon resonance (T-LSPR) spectroscopy [1, 2].
Some earlier works of the inventors have shown that ultrathin (typically not exceeding 10 nm nominal thickness) Au island films vapor-deposited onto transparent substrates display a SP extinction peak at 550-800 nm. The shape, intensity and position of the peak depend on the island morphology, determined by the evaporation conditions and post-deposition treatment [3]. Changes in the SP extinction band (intensity and wavelength of maximum absorbance) resulting from binding of various molecules to the Au islands were measured in the transmission mode [2]. A linear relationship existing between the surface coverage by adsorbing molecules, either bound directly to the Au or through a receptor layer, and the change in the SP intensity and wavelength [2], is useful for quantitative sensing. The T-LSPR spectroscopy was shown to be widely applicable, with a sensitivity which depends on the film preparation conditions [4].
WO 2002/068943, assigned to the assignee of the present application, discloses chemical detection and quantification methods and apparatus employing optical properties of ultra-thin metallic films. According to this technique, detecting changes in surface plasmon intensity of an ultra-thin metallic film provides a quantitative indication of an adsorbed or non-adsorbed chemical substance.